Where do I get my Nokia Lumia 1520 repaired in US

I have a Nokia Lumia 1520 ATT version which is now unlocked and out of warranty. It has microphone issues in normal calling mode. I have to either use speaker mode, bluetooth, or headset to talk which is not very convenient all the time.

I went to MS website but did not get any info on where to get it repaired.
If someone already went thru this process and knows the cost of replacing microphone that would be great!
Any help would be greatly appreciated.